Once all rows are planted, gently scatter more of the seed-starting medium to cover the seeds (unless the packet specifies leaving seeds uncovered). Water the surface with a gentle mist and cover the flat with a plastic dome or sheet of clear plastic to maintain moisture.
Some seeds germinate best in cool soil, and others in warm; check the seed packet for temperature requirements. If the seeds demand warmth, place the flat near a heat vent or on an electric heat mat to warm the soil. Watch daily for emerging seedlings, and as soon as they appear, remove the plastic cover and move the flat to a sunny window or grow lights. At this point, many types of seedlings prefer cool growing temperatures around 65ºF.
When seedlings have grown their second set of leaves, they are ready to be transplanted to individual containers. Use a chopstick or letter opener to lift and loosen the soil beneath a row of seedlings. Gently tug the seedlings apart. Hold the seedlings by their leaves instead of their stems, which are more easily damaged by rough handling. Don’t worry if some roots are torn in the process, or if most of the potting mix falls away from the roots.
Add some potting mix to the bottom of a container (for the tomato seedlings shown, we’re using 3-inch-square pots). Center the seedling in the pot, then add more soil to fill.